In natural conditions, blueberry plants flower in spring, the fruit thereof matures in summer, and the plants shed leaves and enter dormancy during autumn and winter.
Specifically, in temperate regions of the northern hemisphere (e.g., Tokyo), flower buds form at distal ends of new shoots (newly extended stems) or at joints (leaf axils) of upper leaves of new shoots by about September in response to shortening of days from about July after the summer solstice, shedding of leaves is completed in November when daytime air temperatures are about 10 to 15° C., and flowering in the following year occurs following dormancy. In a normal life cycle, leaf buds begin to sprout after flowers have bloomed and scattered (around April), or at the same time as flowering, leaf buds extend into leaves and stems to form new shoots, and as the new shoots grow, fruit flesh forms/matures around seeds in approximately 60 days, and harvest occurs at the beginning of summer (June). Specifically, blueberry is a deciduous fruit tree that can be harvested only once per year. The same life cycle is observed in so-called one-season-bearing fruit trees such as cherry, plum, peach, and apple.
Consequently, the harvest period for blueberries under natural conditions (sunlight only) in Japan is from June to September at best, considering a combination of varieties having different fruit maturation periods and other characteristics. Even in the case of protected cultivation using greenhouses, the harvest time is from mid-April to early June, and a hiatus in domestic harvesting and exporting of blueberries occurs from October to March of the following year (off-season). The supply of fresh blueberries is therefore insufficient for demand from October to May of the following year, and blueberries imported from Chile and other southern-hemisphere countries are expensive.
Patent Reference 1 from the present inventors discloses a cultivation method whereby harvesting even in the off-season is made possible through use of a blueberry variety having a relatively short chilling requirement of about 100 to 500 hours for breaking dormancy. The method disclosed in Patent Reference 1 includes a step for inducing flowering by placing the blueberry plant in an environment having a temperature higher than the dormancy-inducing temperature thereof after the formation of flower buds (about September under natural conditions in Tokyo), and a step for then (starting in about November) placing the blueberry plant in conditions having a longer light period than during induction of flowering. This method makes it possible to cause continuous flower bud formation/flowering/fruiting to occur while fruit is harvested throughout the off season (from December to about July of the following year in this case).
However, although the technique disclosed in Patent Reference 1 enables continuous flower bud formation, flowering, and fruiting, the amount of growth of new shoots is suppressed when large amounts of fruit are produced, leading to reduced leaf area. Fruit size is thereby reduced, and large yields cannot be expected.
Consequently, when the method of Patent Reference 1 is applied, although flowering and fruiting are repeated and the period for such reproductive growth is dramatically lengthened, vegetative growth of foliage is inadequate, and tree vigor is therefore difficult to maintain. Specifically, in a blueberry or other one-season-bearing fruit tree, sprouting of leaf buds and growth of new shoots begin at the same time as flowering or after flowers have bloomed and scattered. Therefore, in the system of Patent Reference 1 for producing repeated flowering after flowers have bloomed and scattered, the vegetative growth period for foliage until the next flowering is extremely short or almost absent. Therefore, in order to achieve more long-term continuous production of blueberries, there is a strong need for sprouting of leaf buds and growth of new shoots to be initiated together with flowering, and also for foliage to be developed. Patent References 2 and 3 disclose systems for controlling the growth environment of plants, but do not suggest methods of control capable of satisfying the above requirements.